The invention relates to methods and apparatus for controlling injuries and the probability of injury in survivable crashes of aircraft, particularly helicopters.
Maintaining an acceptably low probability of spinal injury in a survivable aircraft crash is an important goal. This is particularly true for helicopters operated by the military. Achieving this goal is influenced by a number of factors, including the size of the seat occupant. In recent years, the range of sizes of seat occupants has widened because the pilot population has become more diverse. For example, females are generally smaller than their male counterparts. As females entered the military pilot population, the minimum seat pan supported weight which required an acceptable spinal injury probability during a survivable helicopter crash was appreciably reduced. At the same time, the maximum weight of the pilot population also rose appreciably. As a result, the ratio between the minimum weight pilot who should be provided acceptable spinal accelerations during the energy absorber stroke and the maximum weight pilot who should be provided acceptable energy absorption stroke distance has been drastically increased.
For many years the need for protecting the spine of the crewmembers in aircraft, particularly in vertical takeoff/landing aircraft, has been recognized. Crashworthy energy-absorbing seats were developed which used fixed level energy absorbers (FLEA) to provide spinal protection to the pilot and copilot of these types of aircraft in use by the United States Armed Services and first went into use in the 1970's. At that time only male pilots were flying in those aircraft and would use these crashworthy energy-absorbing seats. As a result the FLEAs designed for these seats were suitable for only a limited range of pilot weights. The force level provided by these FLEAs was set to provide the average weight male pilot (50th-percentile) during a crash event with a controlled deceleration of about 14 G, i.e., 14 times the acceleration of earth gravity. These crashworthy energy-absorbing seats were to provide a nominal 14 inches of travel of the seat pan, which supports the seat occupant during the energy absorber stroke, before the seat would contact the cockpit floor.
However, in a crash event a smaller seat occupant (having a lower weight) will experience a higher deceleration during the FLEA stroke. This will result in a higher probability of spinal injury. Further, in a crash event a larger seat occupant (having a higher weight) would experience a smaller deceleration with a resulting larger travel during the FLEA stroke. Since this larger seat occupant would be expected to have adjusted the seat pan downward from its nominal position to have proper eye level in the cockpit, this larger travel of the FLEA would probably result in the seat pan impacting the cockpit floor with enough residual velocity to severely injure the spine of the heavier seat occupant.